PCB Layout Enables Lowest Thermal Resistance

Printed-Circuit Board (PCB) layout is critical to both thermal and RF performance. It is therefore crucial that designers consider the heat source as well as how the heat will be removed before the electromechanical design is finished. Texas Instruments, Inc. (Dallas, TX) offers a white paper on this subject titled, "Junction Temperature of TRF1123/TRF1223 and Recommended PCB Layout Guidelines." Specifically, this seven-page document describes the PCB layout guidelines for optimal thermal performance while reporting on junction-temperature measurements for the TRF1123 and TRF1223 monolithic microwave integrated circuits (MMICs). It concludes by calculating the failure rate of those MMICs using measured junction temperatures and known reliability data for the MESFET process.

The TRF1123 and TRF1223 are galliumarsenide (GaAs) MMICs packaged in a 5 5-mm, 32-lead, leadless-plastic-chip-carrier (LPCC) package. An 8-mil-thick copper base (paddle) extends through the bottom of the package. The package is designed to solder-reflow the paddle to a PCB groundpad, thereby achieving low thermal resistance and superior RF performance.

For most applications, these MMICs are mounted in environmentally sealed housings and heat removal is by conduction only. There is no internal airflow. Heat flow is entirely through the package paddle, the mounting PCB, and ultimately through a metal heatsink to ambient temperature. For this configuration, the lowest thermal resistance can be achieved using the minimum number of thinnest PCB layers and the thickest copper possible. Practical PCB layout dictates four layers for trace routing and shielding. In the paper, TI illustrates and describes the PCB cross section that it recommends.

The application note offers a lot of advice regarding the layout of these PCBs. For example, there should not be any solder mask on the PCB's bottom side. Any solder mask in that location will impede heat flow while increasing thermal resistance. A variety of measurement results also are provided. Using the Arrhenius Equation, median time to failure (MTTF) is estimated for both MMICs. For a continuous channel temperature of 175°C, the median lifetime is greater than 1.6 million hours (182 years). The note concludes by showing how to calculate maximum base temperatures and power dissipation. The document delivers PCB layout guidelines for the TRF1123 and TRF1223 that provide the lowest thermal resistance when the heat path is through the PCB and into a metal heatsink.